The present invention generally relates to radiotelephone communication systems and more specifically relates to a method and apparatus for reducing the potential interference from a mobile or portable radiotelephone after being handed off from one cell to another in a cellular radiotelephone system.
Mobile radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting with high power to a limited number of mobile or portable units in a large geographic area. Mobile or portable transmissions, due to their lower transmission power, were generally received in previous systems by a network of receivers remotely located from the central site and the received transmission was subsequently returned to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the limited number of channels available.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the channels in a metropolitan area by dividing the radio coverage area into smaller coverage areas (cells) using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166; 4,485,486; and 4,549,311, each assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a seven cell repeating omnidirectionally illuminated cell pattern shown in FIG. 1. In this pattern, radio frequency energy is transmitted from and received by a plurality of centrally located fixed stations and reuse of frequencies is accomplished in a pattern of cells such as that shown shaded in FIG. 1.
An alternative cellular pattern, FIG. 2, depicts a corner illuminated cell system in which 120.degree. antennas are employed to illuminate the interior of a cell from three of the vertices of a hexagonal cell. (Although cell systems are conventionally shown as regular hexagonal patterns, such regularity is rarely achieved in practice).
Another pattern, FIG. 3, depicts a center illuminated cell system in which the cells are further subdivided into sectors. The sectors are illuminated by 60.degree. antennas as illustrated in FIG. 3. A center illuminated sector cell system is further described in U.S. Pat. No. 4,128,740 and assigned to the assignee of the present invention. Thus, a large number of channels can be made available in a metropolitan area and the service provided thereby can appear to be identical to a standard wire line telephone.
A cell system typically utilizes one duplex frequency pair channel in each cell (a signalling channel) to receive requests for service from mobiles and portables, to call selected mobiles or portables, and to instruct the mobiles or portables to tune to another channel where a conversation may take place. This signalling channel is continuously assigned the task of receiving and transmitting data to control the actions of the mobile and portable radios. If the cell is sectorized as shown in FIG. 3, specialized receivers have been developed to enable the inputs from six 60.degree. antennas to be combined for instantaneous reception over the sectorized cell coverage area. One such specialized receiver is described in U.S. Pat. No. 4,369,520 assigned to the assignee of the present invention.
Since the cells may be of relatively small size, the likelihood of a mobile or portable travelling between sectors or out of one cell and into another cell is high. The process of switching the established call from one sector or from one cell to another is known as handoff. Handoff generally requires specialized receiving equipment such as a "scanning" receiver which can be instructed to tune to any of the channels in use in any of the sectors of the cell to measure the signal strength of each active mobile or portable. If the measured signal strength is below a predetermined level, cellular control equipment determines the availability of other channels in other sectors of the same cell or in neighboring cells and composes an instruction to the mobile or portable commanding it to tune to the new channel.
As cell diameters become smaller or as cellular systems become busier, the likelihood of a mobile or portable being handed off to a target cell while transmitting at a power level either too strong or too weak for the conditions in the target cell becomes greatly increased. This is complicated by the fact that in order to minimize interference on channels which are reused elsewhere in the cellular system, operational parameters are adjusted in some cells such that mobiles and portables operating in these cells are maintained at reduced power levels. Handoff thresholds are established to cause handoffs at this reduced power level from these reduced power cells while neighboring cells may be operating with full power levels and may handoff at full power level. Thus, if the transmission power level from the mobile or portable being handed off from one cell to another is not selected properly, the mobile or portable may become a source of interference to other radiotelephone subscribers operating on the same or adjacent channels